Here's why:

* High Energy Density: Triglycerides contain a high proportion of carbon-hydrogen bonds, which store a large amount of energy. They provide more than twice the energy per gram compared to carbohydrates or proteins.

* Efficient Storage: Triglycerides are stored in a nearly anhydrous (water-free) form, meaning they don't add much weight to the organism due to water content.

* Insulation: Fat layers also provide insulation against cold temperatures.

While other lipids like phospholipids and steroids have important functions in the body, triglycerides are the primary energy storage form.